If the instructor is also the business owner, I think a tip is considered unecessary and inappropriate. Otherwise, a paid employee would be thrilled to receive $10/person - an amount I would pay, more or less, depending upon my satisfaction level.
A sailing instructor I know, said it's actually rare to receive tips from students . . . have you ever tipped your college professors?
How do you think I graduated? You have to tip at the beginning of the semester though.